Title:
APPARATUS AND METHOD OF CHARGE-PARTICLE SPECTROSCOPY FOR CHEMICAL ANALYSIS OF A SAMPLE
United States Patent 3766381


Abstract:
An area of a sample is irradiated with electromagnetic radiation so as to release electrons from the area. The electrons are focused to form an image of the sample in an image plane. An apertured image plate is positioned in the image plane to permit only those electrons from a small region of the emitting area to pass. Electrons passing the aperture are energy analyzed. The result is the equivalent of using a microprobe to irradiate the small region. The particles may be retarded before they enter the analyzer, in order to increase sensitivity. A focusing system of five or more lens elements is shown for permitting the analyzer to be used with or without the lens plate without repositioning the system lenses.



Inventors:
WATSON J
Application Number:
05/250893
Publication Date:
10/16/1973
Filing Date:
05/08/1972
Assignee:
WATSON J,GB
Primary Class:
Other Classes:
250/295, 250/305
International Classes:
H01J49/48; (IPC1-7): H01J37/00
Field of Search:
250/49
View Patent Images:
US Patent References:



Other References:

"Electron Optics" Klemperer Cambridge Univ. Press 1953..
Primary Examiner:
Lawrence, James W.
Assistant Examiner:
Anderson B. C.
Claims:
What is claimed is

1. Apparatus for charged-particle spectroscopy for chemical analysis of a sample comprising:

2. Apparatus according to claim 1 wherein said focusing means comprises at least four mutually electrically insulated lens components through which the particles pass in turn, and means for applying respective potentials to said components.

3. Apparatus according to claim 1 wherein said energy analyzer comprises two concentrically mounted, mutually electrically insulated metal hemispheres, and means for applying a voltage between them.

4. Apparatus according to claim 1 wherein said means for irradiating the sample comprises an X-ray source.

5. An apparatus as defined in claim 1 wherein said screening means comprises an apertured plate member which extends in said image plane, and said moving means includes drive means coupled to said apertured plate member for moving said plate member in a direction parallel to said image plane.

6. The apparatus of claim 5 wherein said focusing means comprises a lensing system having at least five lens components.

7. A method of chamical analysis of a sample comprising:

8. A method according to claim 7 wherein the step of analyzing the energies of the particles comprises deflecting the particles in an electrostatic field, positioning collector means to collect deflected particles whose energies lie within a limited range of values, and sweeping the strength of said electrostatic field through a range of values so as to sweep said collector means through the energy spectrum of the charged particles.

9. A method according to claim 7 including the further step of retarding the particles before they enter the analyzer whereby they enter the analyzer with lower energies than they had on being released from said sample.

10. A method according to claim 8 including the step of switching between a first mode of operation in which the particles are retarded, whereby they enter the analyzer with lower energies than they had on being released from said sample, and a second mode of operation, in which the particles are substantially not retarded before they enter the analyzer.

11. A method according to claim 10 wherein said step of switching is performed by altering electrical connections only.

Description:
CROSS REFERENCE TO RELATED APPLICATION

This application is related to U. S. application for letters Pat. No. 119,327 filed Feb. 26, 1971 by Brian Noel Green and John Merza Watson, entitled "Electron Retardation."

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and apparatus of charged particle spectroscopy for chemical analysis known as ESCA. In this class of analysis, a sample is irradiated with a suitable source of electromagnetic radiation such as X-rays or ultravoilet. This irradiation causes electrons to be released by the sample.

2. Description of the Prior Art

With the apparatus of the referenced patent application, emitted electrons pass through an electron lens system into an electrostatic analyzer. The amount a given electron is deflected as it passes through the electrostatic analyzer is a function of the energy of the electron. Analysis is achieved then by scanning the potentials applied to the analyzer so that electrons of differing energies are sequentially focused on a detector.

Typically with an ESCA instrument, when the irradiation source is X-rays, a larger surface area of the sample is irradiated. Since X-rays are not, at least at the present state of technology, capable of being focused, it has been necessary to use a collimator to absorb all rays other than those traversing a small path if a very small and localized spot is to be analyzed. Relative movement of the collimator and X-ray source with respect to the specimen is necessary to select the desired spot on a specimen that is to be irradiated by this collimated beam. With this relative movement being required within the high-vacuum conditions in a source chamber, it will be appreciated that it is difficult to be certain just what portion of the sample is being irradiated. In addition, it is difficult to manipulate the device to return to a specific spot afer one has actually moved the beam away. Further, the confinement of the X-ray source, its collimator and the specimen within an evacuated chamber creates mechanical and manipulation problems which are difficult to overcome.

The energy of X-rays is a function of what is known as the inverse square law. That is, the strength of X-radiation in any given plane is reduced, as compared to the energy of the X-rays at the source, in proportion to the square of the distance between the source and the plane. The presence of a collimator between the X-ray source and the specimen obviously prevents the source and the specimen from being positioned as closely together as might otherwise be possible. The needed space between the source and the specimen for a collimator obviously prevents maximization of the energy of the X-rays which strike the specimen; and because of the inverse square law, the resultant energy reduction is substantial.

There have also been proposals to use a fine probe, known as a microprobe, in the analysis of a small area of a sample in an ESCA instrument. A microprobe, however, exhibits many of the same problems discussed above with respect to a collimated X-ray beam.

SUMMARY OF THE INVENTION

The present invention provides apparatus for spectroscopy with charged particles such as electrons or ions. With this invention it is possible to analyze charged particles from a small region of a sample without the necessity for using a fine probe or collimated beam of radiation.

According to the present invention, an extended area of a sample is irradiated so as to release charged particles from the whole of that extended area. A charged-particle focusing system is provided for forming a charged-particle image of said extended area in an image plane. An apertured image plate is positioned at the image plane for obstructing the passage of charged particles other than those emitted from a small region. An energy analyzer is provided for analyzing the energies of the particles passing through said image plate.

The invention provides, in effect, a "virtual microprobe" since the apparatus analyzes only those charged particles originating from the small region. The effect is the same as if the sample were irradiated with a fine probe of radiation to cause particles to be emitted only from that small region.

In addition, a focusing system of five or more lens elements is provided. This permits the apparatus to be used, with the lens plate removed, as an analyzer of the type described in the referenced application without the need for repositioning the lens elements.

Accordingly, it is the principal object of the invention to provide a method and apparatus for charged-particle spectroscopy which permits analysis of charged particles from a small region of a sample.

This and other objects of the invention will become apparent from the following description of various embodiments of the invention, by way of example with reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of apparatus for electron spectroscopy for chemical analysis of a specimen arranged to operate in a first mode;

FIG. 2 illustrates another mode of operation of the apparatus of FIG. 1; and,

FIGS. 3, 4, and 5 illustrate three modifications of the arrangement of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a sample 1 is mounted on a sample holder 1A within a sample chamber, a small portion of which is shown at 7. The sample is positioned for irradiation over an extended area by an X-ray beam 2 emitted from an X-ray target 3.

The irradiation of the sample causes the sample to release charged particles in the form of electrons. Some of the emitted electrons are represented by the dashed lines 4. These electrons pass through an aperture in a source slit 5. The slit and the sample are at ground potential.

After the electrons pass through slit 5, they enter an electron optical focusing system 6. The system 6 has four cylindrical lens components 7-10. The lens component 7 is formed by a part of the sample chamber, and is also at ground potential. The remaining three components 8-10, respectively, are separate tubular members. At least some of the elements are electrically insulated from the grounded parts of the spectrometer in each mode of operation. The components 7-10 are tubular elements having axially aligned through passages of cylindrical configuration. As shown, the passages are each of the same diameter. The passages are axially aligned with the slit 5.

In operation, when suitable potentials are applied to the components 8-10, as will be described, the electrons 4 are focused to form an electron-optical image of the irradiated area of the specimen 1 in an image plane 11. The image in the plate 11 is magnified, with respect to the sample 1, by a factor substantially greater than unity.

An apertured image plate 12 is provided. The upper surface of the image plate 12, as viewed in the drawings, is disposed in the plane 11. The plate is apertured to allow some of the electrons forming the image to pass while obstructing the passage of other of the electrons 4. Thus, the image plate permits only those electrons emitted from a small region of the sample 1 to pass. The aperture in the source slit 5, on the other hand, is wide enough to pass electrons from a larger specimen area than are permitted to pass through the image aperture in the plate 12.

The angular divergence of the electrons passing through the image plate 12 is restricted by means of an angular aperture 13 situated within the lens component 10 and electrically connected thereto.

A hemispherical electrostatic analyzer 14 is provided. The analyzer 14 has an entry plane which coincides with the image plane 11. The analyzer has a pair of concentrically mounted metal hemispheres 15, 16 which are mutually electrically insulated. In operation, a voltage is applied between the hemispheres 15, 16 by means of an analyzer voltage supply 17. As will be described in greater detail, the outer hemisphere 15 is at the negative potential with respect to the inner hemisphere 16 so as to cause electrons passing through the image plate 12 and entering the space between the hemispheres to follow curved trajectories such as the trajectory represented by the dashed lines 18.

The electrons which pass through the analyzer form an electron-optical image of the image aperture in the plate 12 in an exit plane 19 of the analyzer. The cross-over point of electrons in the exit plane is diametrically opposite to the image aperture. It will be appreciated that in fact a large number of overlapping images of the aperture are formed in the exit plane 19, since electrons with greater energies will follow trajectories of greater radius of curvature and therefore be focused in different positions.

An exit slit 20 is provided. The exit slit has a surface disposed in the exit plane 19. The exit slit serves to select electrons having energies within a certain limited range of the energy spectrum by blocking the passage of non-focused electrons. The selected electrons are detected by an electron multiplier 21 the output of which produces an output signal proportional to the detected electrons.

In operation, the voltage applied from the supply 17 between the hemispheres 15, 16 is swept through a predetermined range of values by means of a scan unit 24. The result of sweeping the voltage is to proportionally vary the limited range of electron energies which may pass through the exit slit 20 and thus scan through the energy spectrum of the electrons.

The output of the counter 23 is fed to the Y-input of an X-Y recorder 25 such as a strip chart recorder. The X-input is fed from the scan unit 24 and is a signal proportional to the analyzer voltage. Thus, the recorder produces a record of the energy spectrum of the electrons from the small region of the sample selected by the image plate 12 and enables the chemical structure of that small region to be analyzed.

The small region of the specimen which is examined is a selected region. Region selection is accomplished by moving the specimen 1 and the image plate 12 relatively. In the preferred arrangement, this relative adjustment is accomplished by moving the lens plate 12. In FIG. 1, a mechanism for moving the lens plate along a path paralleling the plane of cross section of FIG. 1 is shown. A comparable mechanism is provided but not shown for shifting lens plate 12 along a path perpendicular to the plane of cross section of FIG. 1.

Referring to FIG. 1, a micrometer feed screw 64 is threaded into a fixed nut 65. The feed screw 64 is connected to a positioning rod 66 by a swivel 67. Threading the micrometer feed screw 64 in and out in the nut 65 shifts the rod 66 to the right or left as viewed in FIG. 1 to shift the aperture to the right or left.

A flat, annular fringe field plate 26 is provided. The fringe field plate 26 is positioned near the base of the analyzer 14, and serves to overcome fringe field effects at the edges of the analyzer 14. The fringe plate has a spaced pair of apertures 27, 28 respectively adjacent and aligned with the image plate and the exit slit apertures. The fringe field plate apertures allow electrons to pass into and out of the analyzer. The plate 26 is held at a potential intermediate the potentials of the hemispheres 15, 16.

It will be appreciated that all the parts of the apparatus through which the electrons pass must be held in a high vacuum. In addition, a paramagnetic screen (not shown), of a material such as mu-metal may be provided to enclose the analyzer 14 and at least a part of the lens system 6. The screen serves to reduce magnetic and electromagnetic perturbation of the electron trajectories by stray fields.

OPERATION

In a first mode of operation, the electron optical focusing system 6 is arranged to retard the electrons by a factor R, in addition to focusing the electrons to form the image in the plane 11. As shown in FIG. 1, the lens component 8 is connected, along with component 7, to ground potential. The components 9, 10 are connected together electrically, and are held at a negative potential.

The focusing of the electrons to form the image in the plane 11 is effected, in this case, by the electric field within the gap 34 between the lens components 8, 9.

The potential of the components 9, 10 is proportional at any given instant to the potential difference between the hemispheres 15, 16 and hence is proportional to the energy of the electrons which are entering the detector 21 at that instant. Furthermore, the potential difference between the hemispheres 15, 16 is applied in such a way that the central one of the electron paths 18 is at substantially the same potential as the final lens component 10. Thus electrons along the central path do not undergo any further retardations or accelerations after passing through the focusing system 6.

In this mode, the electrons entering the electron multiplier 21 at any point of the spectrum scan, are retarded by the predetermined factor R, which is constant throughout the spectrum. This retardation allows the analyzer to have a higher resolving power and/or allows a greater magnification to be used, thereby increasing the sensitivity of the instrument.

FIG. 1 shows one way in which the necessary potentials can be applied to the lens components 7-10, the hemispheres 15, 16 and the fringe field plate 26 for the first mode of operation. The voltage from the supply 17 is applied to the hemispheres 15, 16 by way of a potential divider, comprising four resistances 29-32 connected in series in that order between the negative and positive terminals of the supply 17. The positive end of the potential divider is grounded and the negative end is connected to the outer hemisphere 15. The inner hemisphere 16 is connected to the common point of resistances 31 and 32. The lens component 8 is connected to the grounded component 7. The lens components 9, 10 are connected together and to a sliding contact 33 on the resistance 30. The fringe field plate 26 is connected through resistor 70 to the common point of resistors 29, 30.

As the output of the supply 17 is varied, the potentials of both hemispheres, and also the potential difference between the hemispheres, vary porportionally to the supply voltage. Similarly, the potential of the components 9, 10 varies proportionally to the supply voltage and by suitable adjustment of the sliding contact 33 this potential can be made substantially equal to the mean potential of the hemispheres, and therefore to the potential of the central electron path 18. In addition, the voltage between the lens components 8 and 9, which is responsible for both the focusing and the retarding effect of the focusing system 6, varies proportionally to the supply voltage. Further, the potential on the fringe field plate 26 is held intermediate the potentials on the hemispheres 15, 16.

Referring now to FIG. 2, a second mode of operation is shown that is suitable for lower energy electrons, e.g. below 100 EV. Here the electron optical focusing system is arranged substantially not to retard the electrons, but only to act as a lens, focusing the electrons in the plane 11.

In this mode, the potential difference between the hemispheres 15, 16 is derived from a potentiometer 38, connected across the voltage supply 17. The center point of this potentiometer 38 is grounded so that, as a result, the hemispheres are held at substantially equal potentials respectively positive and negative with respect to ground. Thus, the central electron path 18 through the analyzer is held at ground potential as the analyzer voltage is swept. The component 10 is connected, along with the component 7, to ground. The intermediate components 8, 9 are connected together and to a tapping point on the potentiometer 38 which can be set at either a positive or a negative potential with respect to ground.

The focusing of the electrons to form the image in the plane 11 is effected, in this case, by the electric fields within the gap 35 between the lens components 7 and 8 and within the gap 36 between the lens components 9 and 10.

In an unillustrated alternative arrangement to that of FIG. 2, the component 9 may be connected to ground, along with components 7 and 10, instead of to component 8 so that only component 8 is connected to the positive or negative potential. In that case, focusing will be effected by the electric fields within the gap 35 and gap 34 between the components 8, 9. Which of these two alternatives is used will depend on the magnification required.

It will be appreciated that switching between these two modes is effected purely by changing electrical connections to the components 7-10, and the hemispheres 15, 16, without any alteration of the mechanical layout of the analyzer.

The image plate 12 may be removable, in which case the apparatus may also be operable in a similar manner to that described in the referenced application, i.e. without the "virtual microprobe." In that event the position of the gaps 34-36 will require position corrections if the instrument is to be operated in all of these further modes.

Referring now to FIG. 3, in a modification of the arrangement of FIG. 1, the problem of positioning is overcome by replacing the focusing system 6 by an electron optical focusing system comprising five (or more) lens components 41-45. By connecting these components to suitable electrical potentials the apparatus can be operated with the image plate 12 in position, in either of the two virtual microprobe modes described above; or with the image plate 12 removed, the instrument of FIG. 3 can be operated in any of the three modes described in the referenced application. The only necessary changes, apart from removing or inserting the image plate 12, to switch between these modes are changes in the applied electrical potentials.

While in the example described above the sample is irradiated with X-radiation, in alternative embodiments of this invention the sample may be irradiated by other forms of electromagnetic radiation, such as ultraviolet light, or by other, nonelectromagnetic radiation, such as electrons. Where the irradiation is, for example, ultraviolet light, the electrons emitted from the sample will in general be of low energies, so that in that case the analyzer will be operated in the non-retarding mode only.

The apparatus can also be operated, with the image plate 12 removed, in a further mode, not described in the referenced application. In this further mode, the electrons are substantially not retarded, but are focused by the focusing system with a magnification substantially greater than unity. This mode is useful, for example, where a low-energy microprobe, such as ultraviolet, is used.

For some purposes, it might be sufficient to design the apparatus so that it could only operate in one of the two "virtual microprobe" modes described above. Referring to FIG. 4, in another modification of the arrangement of FIG. 1, the focusing system 6 comprises only three lens components 51-53. Such an arrangement can operate in the second virtual microprobe mode, in which the focusing means acts to focus the electrons but producing substantially no retardation. In this case, the first and third of the lens components 51, 53 are connected to ground, while the second component 52 is connected to a suitable potential, either positive or negative. This arrangement is only suitable for analyzing low energy electrons, and may be used, for example, in conjunction with an ultraviolet source of irradiation.

Referring to FIG. 5, in a further modification of the arrangement of FIG. 1, the focusing system 6 comprises only two lens components 61, 62. Such an arrangement can operate only in the first virtual microprobe mode, in which the focusing system acts both to focus the electrons and also to retard them. In this case, the first component 61 is connected to ground, and the second component 62 is connected to a suitable, negative, retarding potential.

It should be appreciated that while the apparatus described herein includes a hemispherical energy analyzer, in other forms of the invention different kinds of energy analyzers may be used. For example, the hemispherical analyzer may be replaced by a cylindrical mirror analyzer such as described in U.S. Pat. application Ser. No. 236,748 filed Mar. 21, 1972 by John Merza Watson under the title "Method and Apparatus for Charged Particle Spectroscopy."

Although the invention has been described in its preferred form, it is to be understood that the present disclosure has been made only by way of example and that numerous changes in details may be made without departing from the spirit and the scope of the invention as hereinafter claimed.